This invention relates to improvements in the method wherein steel alloys are melt extruded to produce fine diameter wire.
Until quite recently, it was not possible to fabricate filamentary structures from metals or metal alloys by the method of melt extrusion. The limiting factor was that the melt viscosity of these materials is so low as to be practically negligible. In other words, the melts of metals and metal alloys are essentially inviscid.
The problem presented by an inviscid melt when attempting to extrude it to form filaments is that the surface tension of the filamentary jet, as it issues from the shaping die, is so great in relation to its viscosity that the molten stream breaks up before sufficient heat can be transferred for conversion to the solid state.
This intractable problem has now yielded to a unique solution as described in U.S. Pat. Nos. 3,216,076 and 3,658,979. In accordance therewith, the nascent molten jet, as it issues from the shaping die, is brought into contact with a gas capable of instant reaction with the jet surface. The result is the formation of a thin film which envelopes the jet surface. This thin film has been found to be capable of holding the jet stream together until sufficient heat can be transferred to effect solidification. For example, fine diameter wire may be formed from aluminum by extruding the melt at an appropriate velocity into an oxygen medium. When the hot jet issuing from the extrusion orifice contacts the oxygen-containing atmosphere, a stable film of melt insoluble aluminum oxide forms almost instantaneously about the peripheral surface of the jet. In essence, a sheath is formed which protects the filamentary jet or stream against surface tension break-up until solidification takes place.
The oxide of aluminum is a solid which is insoluble in the non-oxidized molten metal. This, of course makes film formation by contact with oxygen below the orifice possible. However, in the instance of ferrous metals, as for example steel, the iron oxide is soluble in the liquid melt. Consequently, a film will not form when a molten jet is extruded into an oxidizing atmosphere.
A solution to this problem is provided in the teachings of U.S. Pat. No. 3,216,076. As disclosed therein, filamentary structures may be formed from metals whose oxides are soluble in the non-oxidized molten metal by alloying them with a minor percentage of a compatible metal whose oxide is insoluble in the non-oxidized molten metal. By compatible metal there is meant a metal or combination of metals having the ability to form an alloy. According to U.S. Pat. No. 3,216,076 metals which may be used for this purpose include aluminum, magnesium, beryllium, chromium, lanthanum and combinations thereof. The particular metal employed is generally present in amounts in excess of 0.5% by weight of the alloy. The upper limit on the quantity of metal which will produce a stable oxide is only determined by the physical characteristics desired in the ultimate filamentary product. The metal most commonly alloyed with steel for effecting film formation when extruding steel melts has been aluminum.
The extension of the capability for producing filaments directly from the melt to metals like steel constitutes an important advance in the art. However, commercial scale practice of this potentially attractive method for producing steel wire has been inhibited by an inability to control the tendency for the orifice to plug during extrusion. Oxidation reactions occurring in the melt prior to extrusion are largely responsible for the partial or complete plugging of the orifice. Contributing to this problem has been a premature oxidation of the second metal used to stabilize the molten stream of steel. As has been noted, aluminum is commonly used for this purpose, and it has not been found practical to maintain the melt oxygen content at the very low levels required for avoiding a premature precipitation of alumina and complexes thereof with the oxides of other metal impurities. Such precipitates form solid inclusions in the melt which accumulate in the orifice area and tend to plug it. Likewise, a similar problem exists with other metals heretofore proposed for alloying with steel to provide a stabilization capability.
It is therefore a principal object of this invention to provide a means for extruding a steel alloy melt to form fine diameter wire in the absence of orifice plugging caused by insoluble oxide formation above the extrusion orifice.
It is another object of this invention to provide a means for controlling the oxygen potential of the melt upstream from the extrusion orifice during the extrusion of molten steel to prevent the occurrence of orifice plugging.
Other objects and advantages will become apparent from a description of the invention which follows: